THE FOREST TENT CATERPILLAR IN ASPEN STANDS - DISTRIBUTION AND DENSITY-ESTIMATION OF 4 LIFE STAGES IN 4 VEGETATION STRATA

The forest tent caterpillar (Malacosoma disstria Hbn.) exemplifies the mobile defoliator that expands in vertical distribution during develo pment from tree canopy to ground. For such insects the problem of esti mating absolute numbers of each life stage is complex and has seldom b een addressed. We investigated distribution of eggs, small larvae, lar ge larvae, and cocoons (pupae) in overstory-tree, high-shrub, low-shru b, and ground strata in stands of quaking aspen (Populus tremuloides M ichx.) in which defoliation ranged from virtually none to complete. Th e study took place in 11 stands in northern Minnesota during 5 yr, and in 5 stands in Upper Michigan during 1 yr. Trees were divided for ins ect counting into vertical crown-thirds, horizontal crown-halves, bran ches, branch parts, and bole. Insects were also counted on entire high shrubs, and in several sizes of low-shrub and ground plots. Eggs and small larvae occurred in the tree stratum regardless of population den sity. Vertical and horizontal distribution in overstory trees shifted through successive life stages and with different population densities . Density and defoliation intensity affected proportions of large larv al and cocoon populations occurring in the three lower strata. Cocoons were maximally dispersed, with up to 97% occurring in high-shrub, low -shrub, and ground strata. Egg-mass numbers were satisfactorily estima ted in trees from a sample of branches and tree dbh. Abundance of all life stages on trees was related to a power of tree dbh which at moder ate to high population densities averaged 2.72 for egg masses, 4.23 fo r small larvae, 2.99 for large larvae, and, when defoliation did not e xceed 60%, 2.33 for cocoons. These relations form the core of models d evised for indirectly estimating absolute density of insects in the tr ee stratum from dbh and stand tables. At moderate to high densities, a bsolute density estimation in the four combined strata produced overal l standard errors whose midrange percentages of estimates were 31 for eggs, 56 for small larvae, 40 for large larvae, and 38 for cocoons. Fo rest tent caterpillar distribution in aspen stands appears determined to a high degree by stand foliar area.